This invention relates generally to an apparatus for high speed analog data recording having high resolution, linearity and low distortion. More particularly, this invention relates to a photonic cathode ray tube comprising a flat photocathode, a small aperture electron lensing system, a set of deflection plates and a phosphor screen.
It is well known that conventional oscilloscopes based on the cathode ray tube (CRT) have great difficulty in achieving multi-GHz bandwidth Performance. As a result, multi-GHz oscilloscopes are very costly, and some may approach a cost of close to $90,000 or more per channel at the present time.
An alternate technology that can provide the desired high bandwidth and low cost per channel is a photonic high speed data recording system known as the high speed multichannel data recorder (HSMCDR). The HSMCDR is based on a high speed electro-optic streak camera, and the system is described in an article by J. Chang et al entitled "Photonic Methods of High Speed Analog Data Recording", Rev. Sci. Instrum., Vol. 56, No. 10 56(10), October 1985. In the approach of the HSMCDR, many channels (up to 40) of optical analog data can be input and recorded by one streak camera. However, in order to realize this low cost or cost reduction per channel (as compared to the conventional oscilloscope) it is necessary to record all, or substantially all, of the channels simultaneously. Unfortunately, the necessity of tying forty or more channels together to achieve economy is a handicap because, often times, it is difficult to find a large number of channels that all require the same sweep rate. Moreover, should the streak camera fail at the critical time, then all forty or more channels of data would be lost.
The use of streak cameras also involves economic considerations Streak cameras are very expensive, perhaps on the order of $150,000. Because of that high cost, they are often used in multichannel form (30-40 channels) to reduce the per channel cost. However, while the use of a multichannel streak camera does reduce cost per channel, the total cost of a 30-40 channel HSMCDR may be in the range of $500,000, thus requiring a large investment to get the low per channel cost.
The technology of the streak camera per se, also suffers from several drawbacks and deficiencies. The conventional streak tube is basically a large aperture (and large field of view) optical system that has pronounced edge distortions, sweep nonlinearity and non-uniformity of photocathode response. As a result, the performance of the streak tube is rather limited and often insufficient. In order to compensate for the lack of edge definition in streak cameras, it is usual to detune the center to enhance the definition at the edges of the lens. This detuning is accomplished in the lens which is a large aperture and large field of view system located about mid-way between the photocathode and the phosphor screen.
Another problem with streak cameras is that they require relatively large photocathodes (on the order of 4-5 cm. in diameter), and such large photocathodes are very expensive. It is difficult to get a uniform coating on a large photocathode surface, so manufacturing process yields are very low, thus resulting in very high final cost.
Because the photocathode and the phosphor screen in a conventional streak camera have slightly curved configurations (to provide equal distance to all points), more desirable crystalline material cannot be used to coat the cathode (because the crystal structure is flat). This is still another drawback of these prior art systems.